<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
</head>
<body>
  <div>This is content, and data is: <span id="data"></span></div>
  <button id="button">click me</button>
</body>
<script>
  var index = 0
  function doRequest () {
    var data = document.getElementById('data')
    data.innerText = ''
    fetch('/data', {
      headers: {
        'X-Test-Cache': index++
      }
    }).then(function (resp) {
      return resp.text()
    }).then(function (text) {
      data.innerText = text
    })
  }
  document.getElementById('button').addEventListener('click', doRequest)
</script>
</html>